There has been known an image data processing device which temporarily stores a large amount of image data in a hard disk, and performs printing on the basis of the image data read out from the hard disk (see Patent Document 1: Japanese Unexamined Patent Publication No. 191101/1994 (Tokukaihei 6-191101: published on Jul. 12, 1994)) and Patent Document 2: Japanese Unexamined Patent Publication No. 98034/1996 (Tokukaihei 8-98034: published on Apr. 12, 1996).
The image data processing device allows for a so-called electronic sorting function in which: even when plural copies of image data corresponding to plural pages are printed, the image data is temporarily stored into the hard disk and the image data is read out in the order of pages so that the plural copies of image data are printed.
However, a processing speed (access speed) at which writing/reading of data is carried out into/from the hard disk is very low, as compared with an access speed to a memory. As a result, there occurs such a problem that: even though a processing speed of a printing engine is made faster, the speed of reading data from the hard disk does not catch up with the processing speed of the printing engine, so that a target printing job ability cannot be achieved.
FIG. 7 is a data flow showing a case where image data is temporarily written into the hard disk and the image data is read from the hard disk so that a printing is performed.
Note that, FIG. 7 is an example corresponding to a case where a maximum processing speed of the hard disk is 60 [Mbyte/sec] and an amount of image data per 1 page is 60 [Mbyte/sec]. Further, note that in a case where the maximum processing speed of the hard disk is 60 [Mbyte/sec], when assuming that the same amount of image data is written into and read from the hard disk at a time, each of the maximum writing and reading process speeds is 30 [Mbyte/sec].
As illustrated in FIG. 7, a CPU stores the image data in a memory and then writes the image data into the hard disk. In this case, a maximum processing speed at which the data is written into the hard disk is 30 [Mbyte/sec].
On the other hand, the printing engine reads out the image data from the hard disk to store the image data in the memory, and then performs a printing process on the basis of the image data thus read out. In this case, a maximum speed at which the data is read out is 30 [Mbyte/sec] as described above. Namely, the printing engine performs a printing at a data processing speed of 30 [Mbyte/sec].
Therefore, a printing speed indicative of the number of printed pages per unit time (e.g. per 1 minute) is found as follows.
Printing speed: 60 [sec/min]÷(60 [Mbyte/page]÷30 [Mbyte/sec])=30 [page/min]
Here, it is assumed that the amount of data to be written into is the same as the amount of data to be read out. If printing is performed while data is only read out at a speed of 60 [Mbyte/sec], then a printing speed is temporarily 60 [page/min], which is twice as large as the above printing speed. However, no printing can be performed after one (1) job has been finished and before image data for a following job is written into the hard disk. This does not allow a total of a plurality of jobs to have a printing speed of 60 [page/min]. Therefore, in a case where a printing speed is defined as a speed at which a printing is continuously carried out for a plurality of jobs, it is necessary to simultaneously carry out (i) reading out of image data for a current printing job from the hard disk and (ii) writing of image data for a following printing job into the hard disk. Further, in a case of repeat printing, data once written into the hard disk is read out, and the printing is carried out based on the data thus read out in accordance with the number of required copies. Accordingly, the amount of data written into the hard disk is not equal to the amount of data read out from the hard disk. However, here, there is assumed a case of one-copy printing in which the largest amount of data is dealt with during writing and reading operation. Accordingly, it is assumed that the amount of data to be written into is equal to the amount of data to be read out.
In this way, a printing speed depends on a processing speed of a hard disk. Therefore, Patent Documents 1 and 2 disclose a technique in which: in order to increase a printing speed, image data is subjected to a compressing process by use of an image compressing device before the image data is stored in a hard disk, and the image data is subjected to a decompressing process by use of an image decompressing device after the image data is read from the hard disk, and then the image data is subjected to a printing process.
FIG. 8 is a data flow showing a case where a compressing/decompressing process is performed. Here, it is assumed that an image compressing device has a compressing speed of 60 [Mbyte/sec]. Further, it is assumed that the image decompressing device has a compressing rate of ⅙ at which image data is compressed. Further, it is assumed that image decompressing device has a decompressing speed of 60 [Mbyte/sec].
As illustrated in FIG. 8, image data which has been subjected to a ⅙ compressing process by the image compressing device is written into the hard disk at a speed of 10 [Mbyte/sec]. Similarly, because the image decompressing device has a decompressing speed of 60 [Mbyte/sec], the image data is read out from the hard disk at a speed of 10 [Mbyte/sec]. The image decompressing device performs a decompressing process and transmits the image data to the printing engine at a speed of 60 [Mbyte/sec]. Namely, the printing engine performs printing at a data processing speed of 60 [Mbyte/sec].
Therefore, when assuming that the amount of image data per 1 page is 60 [Mbyte/page] as with FIG. 7, a printing speed is found as follows.
Printing speed: 60 [sec/min]÷(60 [Mbyte/page]÷60 [Mbyte/sec])=60 [page/min]
This shows that the printing speed is increased as compared with an arrangement in which no image compressing device and no image decompressing device are provided.
Note that, in Patent Document 1, in order to reduce the amount of data flowing in an internal bus, image data is sent from the image decompressing device to the printing engine not via the internal bus but via a special data bus. Further, according to Patent Document 2, in a case of using a plurality of image compressing devices and image decompressing devices, a compressing method and a compressing rate are specified with respect to each image compressing device so that each image compressing device generates encoded data having substantially the same size. This allows for preventing a reduction in a speed at which specific encoded data is written into the hard disk and in a speed at which specific data is read out from the hard disk.
The above technique is based on the premise that each of the compressing speed and decompressing speed is higher than a target printing processing speed. In other words, the limit of the performance of the printing process is determined based on the performance limits of the image compressing device and the image decompressing device.
Particularly, recently, as a printing engine gets speedy, each of processing speeds of an image compressing device, an image decompressing device, and a hard disk is becoming a bottleneck with respect to the processing speed of a printing apparatus.
One countermeasure against the bottleneck is to use an image compressing device, an image decompressing device, or a hard disk, each of which can carry out a process at a higher speed, or to simultaneously use at least two which are selected from a group consisting of the image compressing device, the image decompressing device, and the hard disk. However, this countermeasure gives rise to an increase in the cost and to technical difficulties in packaging and controlling.